Stacking Teleconverters

An Unscientific Evaluation with a Canon EOS 300D

A while ago, I bought a Jessop 2x teleconverter. This is a relatively cheap device which doubles the magnification of an SLR camera lens by introducing extra optics between the lens and the camera body. Unlike the more expensive models, these are quite compact and easy to carry around.

A Jessop 2x teleconverter (Canon EOS fit).

Recently, I was fortunate enough to stumble upon an eBay auction where the seller had priced one of these up for only £10. Bargain! Even though I already had one, I could not resist and ended up buying a second, identical 2x teleconverter.

After buying it, I suddenly realised, "What am I going to do with it?" My first thought was that I could resell it on eBay and make a significant profit. But then I had always wondered what happens when you try to stack two 2x teleconverters together...

Stacking teleconverters

Based on previous experiences, I didn't think this was going to work. I have a cheap 650-1300mm manual focus zoom lens which didn't like working with my Jessop 2x teleconverter at all - it filled the viewfinder with funky purple circles and the camera reported an error when the shutter button was pressed. Nonetheless, for £10 I thought there was nothing to lose. To my surprise, both 2x teleconverters stacked together without any problems on my everyday 28-105mm lens.

Two stacked Jessop 2x teleconverters
Two Jessop 2x teleconverters stacked between a Canon 300D and Canon EF 28-105mm f/3.5-4.5 USM Mk I lens.

I thought I would take some photos, both with and without the 2x teleconverters, to compare the quality of the results. I set up the camera on a tripod with a standard Canon EF 28-105mm f/3.5-4.5 USM Mk I lens. The aperture of the lens was set to f/11 throughout and ISO 100 was selected.

I planned to take several photographs of the same object with a variety of focal lengths:

Photo	Lens Focal length	Number of 2x Teleconverters	35mm Equivalent Focal Length*
1	28mm		45mm
2	105mm		168mm
3	28mm	1	90mm
4	105mm	1	336mm
5	28mm	2	179mm
6	105mm	2	672mm

* The Canon EOS 300D has a smaller CCD than a 35mm frame, so there is a focal length multiplier of 1.6x to take into account when working out the 35mm equivalents above.

On paper, this looks quite interesting. By stacking together two of these 2x teleconverters, you can turn a standard 28-105mm lens into a 179-672mm zoom. Even when stacked, the two 2x teleconverters are shorter than the lens. It all sounds quite impressive so far, but how do they perform?

Performance comparison

To compare the performance of 2x teleconverter stacking, I took a single 6-megapixel photo for each of the configurations described above. 100% size 300x300 centre sections are shown below:

Photo 1 (28mm)
The For Sale sign is a bit overexposed due to the darker surroundings (not in shot).

Photo 2 (105mm)
This photo has a much better exposure. This is using the Canon lens, without any 2x teleconverters yet. Reasonable sharpness for a non-pro lens.

Photo 3 (28mm + 2x teleconverter)
Using a single 2x teleconverter, sharpness isn't too badly affected at this focal length.

Photo 4 (105mm + 2x teleconverter)
The image is slightly softer when zoomed in all the way with a single 2x teleconverter.

Photo 5 (28mm + stack of two 2x teleconverters)
With two 2x teleconverters, the image appears noticably washed out and brighter than the previous images.

Photo 6 (105mm + stack of two 2x teleconverters)
Zoomed all the way in with two 2x teleconverters. The softness is definitely starting to show a bit more now.

Comments

Adding a 2x teleconverter reduces the maximum aperture by 2 stops, meaning that a quarter of the light reaches the CCD. Incidentally, this is why they make 1.4x teleconverters, as 1.4 is approximately the square root of 2 and reduces the maximum aperture by 1 stop. Anyway, I digress - stacking two 2x teleconverters together reduces the light levels to 1/16, which is enough to stop autofocus working in some cases.

I had to manually focus in Photo 4 and Photo 6. All other photos used autofocus. My manual focussing abilities shouldn't affect the results significantly, as the aperture was set to f11, giving a reasonable deep depth of field.

Photo 6, taken at 105mm with two stacked 2x teleconverters, is noticably soft. Using stacked 2x teleconverters clearly reduces the sharpness of the image, but then it is allowing you to get closer to the action. I suppose that depends how exciting you find a For Sale sign :)

Internal reflections?

Let's compare the full size Photo 2 (left) and Photo 5 (right). They are taken at approximately equal 35mm-equivalent focal lengths, but Photo 2 uses the plain lens, while Photo 5 uses two 2x teleconverters. Notice how washed out and bright the image appears. This is probably caused by internal reflections created by the 2x teleconverters.

When comparing Photo 2 and Photo 5, also notice how the items in the foreground are more in focus in Photo 5. This is because the 2x teleconverters are "dumb" and simply relay the electrical messages to and from the camera without doing anything clever. So although the camera reported a lens aperture of f11 in Photo 5, the actual aperture was 4 stops lower due to the reduced amount of light being let in by the 2x teleconverters. The Canon 'L' range of teleconverters (Extenders) do clever things in order to report the true aperture, but they are only compatible with certain Canon lenses.

So is it worth using a 2x teleconverter?

Good question! I'm not going to answer that one, because it depends on your style of photography. However, I will answer the following question: Is it worth stacking two 2x teleconverters?

I shall compare the two 105mm shots taken with one and two 2x teleconverters. The photo taken using a single 2x teleconverter (left) was scaled by 200% so it could be compared directly with the photo taken with two 2x teleconverters (right). There is not a huge difference in quality, even after the single-teleconverter image has been scaled up by 200%.

Conclusions

If this level of detail is important to you and you really need to get closer to your quarry, then I would recommend experimenting with multiple 2x teleconverters. They are certainly a [much] cheaper alternative to buying a bigger lens. If quality is not too much of a concern, you might be better off carrying around just one 2x teleconverter and using the computer to scale your images afterwards. Having a second 2x teleconverter seems to ruin the image slightly by increasing the internal reflections inside the lens. Of course, if you shoot with film, then perhaps having two 2x teleconverters is more convenient.

I think I'll be hanging onto my second 2x teleconverter for a while yet.

All photos copyright Paul Mutton.

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